D-Day is a term used in the military to mean the actual day a major operation or event is to begin. The days leading up to a D-Day are called D-1, D-2, D-3, and so on. The days after a D-Day are D+1, D+2, D+3, and so on.

There were many days in military history that were called D-Day. The most famous D-Day was June 6, 1944 when the biggest amphibious (from the sea to the land) attack in military history took place in Normandy during World War II. This attack was codenamed Operation Overlord. The Allies (Britain, Canada, and America) attacked Europe occupied by Nazi Germany. It was very important for the Allied success in World War II, though over 3,500 men were lost while attacking the beaches. After that, about two months later, after very hard fighting and many more killed, the Allies won. They freed France from the Germans. Less than one year later Germany surrendered. June 6 is very important to many people because of what happened.

Many people think D-Day stands for Decision Day, but it does not. The first letter is just to show the importance of the day.